I would like to use these with systemtap to gather
min/max/avg/variance data for gc_*_continue functions.

I prefer to use systemtap/dtrace and not modify or load extra
Ruby code to use built-in tracing. Systemtap also has aggregate
functionality built in for @min/@max/@avg and @hist_log for
generating histograms:

Red Hat-based systems should be similar; and most of the
systemtap team works for Red Hat. There's also dyninst support
which won't require special privileges (or kernel context
switch), but current Debian stable doesn't enable it, yet.

was a bit strict since it requires the path of executable.
Now I favor "-x $PID" or "-c $CMD" so I can use:

probe process.mark("foo")

There is also "systemtap-doc" package
I look at docs + *.stp examples in their git tree:

git://sourceware.org/git/systemtap.git

(*) I never tested `stap-prep' myself since I build my own kernels
from source. For people like me who run the latest kernels;
be prepared to backport patches from systemtap.git or
build+install from that yourself. Users of distro-provided
kernels get things working out-of-the-box in my experience.

Since I always build my own kernels and usually run the latest
kernel, systemtap remains a pain because it relies heavily on
kernel internals. Given that, systemtap should probably be like
perf(1) and be maintained/distributed with the kernel (and the
default prefix should be $HOME :P)